Esters of Diphenic Acid

II. Synthesis of Fatty-Aromatic Esters of Diphenic Acid

L. P. Kulev, R. N. Gireva, A. P. Belyaeva

Among the derivatives of diphenic acid, esters are undoubtedly of practical interest, since they may find application as plasticizers, insecticides, repellents, and herbicides. From this standpoint, the esters of diphenic acid have been studied very little.

The present communication describes the synthesis of arylalkyl esters of diphenic acid.

All the esters were obtained according to the scheme

\[ \text{diphenic anhydride} \ \xrightarrow{\ \mathrm{AlkOH}\ }\ \mathrm{HOOC{-}C_{12}H_8{-}COOAlk} \ \xrightarrow[\ -\mathrm{H_2O}\ ]{\ \mathrm{AlkOH}\,(\mathrm{POCl_3})\ }\ \mathrm{AlkOOC{-}C_{12}H_8{-}COOAlk}. \]

By boiling diphenic anhydride with an excess of an aliphatic alcohol, the acid ester was first obtained [1], which was then esterified with phenols [2] by heating to \(125\text{–}130^\circ\) for 3–4 hours in the presence of phosphorus oxychloride.

When the reaction was carried out under these conditions, it was found that the monoesters of diphenic acid are capable of dehydrating and isomerizing into esters of fluorenonecarboxylic acid

\[ \mathrm{HOOC{-}C_{12}H_8{-}COOAlk} \ \xrightarrow[\ -\mathrm{H_2O}\ ]{\ \mathrm{POCl_3}\ }\ \mathrm{AlkOOC{-}fluorenone} \]

colorless

\[ \mathrm{HOOC{-}C_{12}H_8{-}COOH} \ \xrightarrow[\ -\mathrm{H_2O}\ ]{\ \mathrm{H_2SO_4}\ }\ \mathrm{HOOC{-}fluorenone} \]

bright yellow

Until now, such a transformation, occurring under the influence of dehydrating agents (concentrated sulfuric acid, anhydrous aluminum chloride), was known only for diphenic acid itself [3] and its anhydride [4].

Transformation into fluorenonecarboxylic acid or its ester is accompanied by the appearance of a bright-yellow color.

The monomethyl ester of diphenic acid passes especially readily into a derivative of fluorenonecarboxylic acid; in some cases, for example during esterification with 2-nitrophenol, it does not

it is possible to obtain the mixed alkyl aryl ester, since the rate of conversion of monomethyl diphenate into the keto-acid ester apparently exceeds the rate of esterification of the acid ester. Similar results are obtained by heating the monomethyl ester of diphenic acid with phosphorus oxychloride in the absence of phenol at a temperature of 125–130° for 4 hours. The monoethyl ester is capable of such a transformation to a considerably lesser extent, as a result of which the latter was also used for the synthesis of phenyl esters.

The esters of 9-oxofluorene-4-carboxylic acid obtained here, like the acid itself, dissolve in concentrated sulfuric acid with a characteristic red coloration [5].

To identify these esters, which arise as by-products, with the same esters synthesized from fluorenonecarboxylic acid, the ultraviolet absorption spectra were recorded on an SF-4 spectrophotometer (Fig. 1).

Fig. 1. Absorption spectra of diphenic and fluorenonecarboxylic acids and their esters in ethyl alcohol:
1—diphenic acid; 2—monomethyl diphenate; 3—fluorenonecarboxylic acid; 4—methyl ester of fluorenonecarboxylic acid, obtained from the acid; 5—methyl ester of fluorenonecarboxylic acid, obtained from the methyl ester of diphenic acid; 6—ethyl ester of fluorenonecarboxylic acid, obtained from the acid; 7—ethyl ester of fluorenonecarboxylic acid, obtained from the ethyl ester of diphenic acid.

As is seen from Fig. 1, the esters obtained by these methods are identical with one another.

In a number of cases, in obtaining mixed diesters, transesterification partially takes place, i.e., replacement of the ethyl radical by an aryl radical, as a result of which the corresponding diaryl ester of diphenic acid is formed. Thus, the synthesis of 4-nitro- (III), 3-nitro- (VI), 2,4-dinitro- (VIII), and 2,4,6-tribromophenyl ethyl diphenates (XI) is accompanied by partial formation of di-(4-nitro- (IV), 3-nitrophenyl (VII),

bis-(2,4-dinitrophenyl) and bis-(2,4,6-tribromophenyl) esters of diphenic acid.

Preparation of the 3-nitrophenylethyl ester is carried out by heating the reaction mixture at 120–125° for 1 hour. The synthesis of 2,4,6-trinitrophenylethyl diphenate is conducted for 5 hours at 140–150°, since at a lower temperature the reaction does not proceed; in this case a small amount of the ethyl ester of fluorenonecarboxylic acid is obtained.

Most of the esters obtained dissolve well in ordinary organic solvents. In alcohol, di-3-nitrophenyl diphenate is considerably less soluble; 2,4-dinitrophenylethyl diphenate and 2,4,6-trinitrophenylethyl diphenate are almost insoluble. The alcohol-soluble esters were recrystallized from it, while the insoluble ones were repeatedly washed with this solvent until a constant melting point was reached.

The ultraviolet absorption spectra of the esters obtained are shown in Fig. 2.

Fig. 2. Absorption spectra of diphenic acid and its esters in ethyl alcohol and dioxane:
1—diphenic acid; 2—monoethyl diphenate; 3—phenylethyl diphenate; 4—2-nitrophenylethyl diphenate; 5—2,6-dinitrophenylethyl diphenate; 6—di-4-nitrophenyl diphenate; 7—2,4,6-trinitrophenylethyl diphenate; 8—2,4,6-tribromophenylethyl diphenate.

Experimental Part

Phenylmethyl ester of diphenic acid (1). 7.1 g of monomethyl ester of diphenic acid and 2.63 g of phenol are heated in a round-bottom flask equipped with a reflux condenser and a stirrer. At 120°, 2.9 g of phosphorus oxychloride is added dropwise to the mixture. The reaction is carried out for 4 hours at 125–130° with constant stirring. After heating is complete, the still-warm reaction mixture, in the form of a thick syrup-like almost colorless liquid, is poured into cold-

Aryl ethyl and diaryl esters of diphenic acid

\[ \mathrm{C_2H_5OOC{-}C_6H_4{-}C_6H_4{-}COOAr} \qquad \mathrm{ArOOC{-}C_6H_4{-}C_6H_4{-}COOAr} \]

Table 3

water. A small amount of a resinous, dark-colored mass remains in the flask. Both products are washed repeatedly with water, with soda solution, and again with water, and are recrystallized from dilute (70%) alcohol. After several recrystallizations, the main product—phenyl methyl ester—forms fine white crystals with m.p. 65–66°.

Found, %: C 75.42; 75.70; H 4.26; 4.40, \(\mathrm{C}_{21}\mathrm{H}_{16}\mathrm{O}_{4}\).
Calculated, %: C 75.90; H 4.82.

The resinous mass, recrystallized from aqueous alcohol—fine yellow crystals with m.p. 130°—is methyl ester of 9-oxofluorene-4-carboxylic acid.

Experimental Part

Phenyl ethyl ester of diphenic acid (II). Prepared analogously to (I) from 21.2 g of monoethyl diphenate, 7.39 g of phenol, and 4.28 g of phosphorus oxychloride. The ester, in the form of a thick oil, is recrystallized repeatedly from aqueous alcohol; white crystals are obtained with m.p. 58–60°.

Found, %: C 75.97; H 5.41, \(\mathrm{C}_{22}\mathrm{H}_{18}\mathrm{O}_{4}\).
Calculated, %: C 76.30; H 5.20.

In this process, a very small amount of ethyl ester of fluorenonecarboxylic acid is formed, in the form of fine yellow needles with m.p. 100–102°.

Substituted phenyl ethyl esters of diphenic acid are prepared analogously from 0.011 M monoethyl diphenate, 0.011 M substituted phenol, and 0.008 M phosphorus oxychloride. The characteristics of the esters and their analyses are given in Table 1.

Conclusions

1. A series of substituted and unsubstituted aliphatic-aromatic and aromatic esters of diphenic acid has been synthesized. UV spectra of the compounds obtained were recorded.

2. The ability of acidic esters of diphenic acid to dehydrate and isomerize into the corresponding esters of fluorenonecarboxylic acid has been established.

REFERENCES

1. R. E. Dean, E. N. White, and D. Me Nell, J. Appl. Ch. 3, 469, 1953.
2. A. M. Berkengheim, Chemistry and Technology of Synthetic Medicinal Products, 419, ONTI, Moscow, 1935.
3. C. Graebe, C. Mensching, Ber. 13, 1302, 1880; C. Graebe, Ch. Aubin, Ber. 20, 845, 1887; Lieb. Ann. 247, 275, 261, 1888; H. Stoble, S. Seydel, Lieb. Ann. 370, 134, 1909.
4. R. Gotz, Monatsh. 23, 29, 1902; H. Pick, Monatsh. 25, 980, 1904.
5. C. Graebe, Ch. Aubin, Lieb. Ann. 247, 261, 1888.